The invention concerns regulation methods and devices intended in particular for negative-pressure devices applied to the maintenance of a liquid in an open reserve, the opening being situated below the level of the liquid, so as to counteract the flow of the liquid caused by gravity.
The invention applies particularly to heads in printing systems of the “inkjet” type.
These print heads consist of a reservoir of ink provided with an opening situated below the ink filling level and means, in particular piezoelectric, for projecting a fine drop of ink through the opening, in the direction of the medium to be printed. It is by projecting a multitude of these fine drops at appropriate locations that the required printing is carried out.
Outside the projection phases, the ink must be maintained in its reserve despite gravity, which causes a tendency to flow through the opening.
One effective solution consists of maintaining the gas present in the reserve above the ink at a pressure less than atmospheric pressure, with the use of a negative-pressure device.
The correct maintenance of the ink inside an inkjet print head requires very precise regulation of the negative pressure in order to prevent the flow of ink through the opening if the negative pressure is too little, or taking in air through this same opening if the negative pressure is too strong.
Such methods and devices for regulating the negative pressure are known in the prior art.
For example the patent U.S. Pat. No. 5,189,438 describes a device in which a vacuum pump is connected to the ink reserve, above the ink level, and is continuously activated. The calibration of the negative pressure is obtained by virtue of a calibrated leakage orifice, also situated above the ink level.
These devices, based on principles of mechanical calibration of an exhausting of the vacuum, do nevertheless have a certain number of drawbacks.
This is because the value of the negative pressure thus obtained depends on mechanical elements and therefore does not have sufficient precision to guarantee optimum maintenance of the ink.
For these same reasons, the surety in functioning of these regulation devices is not satisfactory since it depends on the cleanliness of the gas conduits and of the exhaust orifice and has too much dependency on the external environment.
In addition, the known devices do not have any adjustment of the set negative pressure value to be maintained in the ink reserve. Changing the set value amounts to physically changing the exhaust orifice by means of an orifice with another calibre, which cannot be carried out dynamically during the functioning of the print head.